Ethylene Glycol

Process Analysis and Modeling

Warm up Project

September 5, 2001
Benjamin Alan Grayson
 Ethylene Glycol

Ethylene glycol has many interesting properties and uses. At room temperature,

ethylene glycol is clear, colorless and slightly viscous liquid. It has no smell, but has a

sweet flavor to it. It is used as antifreeze and a deicing fluid for vehicles. Also, it is used

to make polyester compounds and is used in the paint and plastic industries as a solvent.

In addition, ethylene glycol is an active ingredient in photographic developing solutions,

hydraulic brake fluids and in inks [1]. When analyzing ethylene glycol, one should begin

with how ethylene glycol is produced, why it should be modeled, and what are some

proposed alternatives to common production techniques.

The major commercial production of ethylene glycol is by the hydration of

ethylene oxide [2]. According to the assignment, the minimum requirements of an

ethylene glycol production plant would be one reactor, two separators, and a distillation

column. However, most ethylene glycol productions plants include ethylene oxide

production plants as well. In order to fully discuss the production, one should look at

how the raw material of ethylene oxide is produced [3].

Diagram 1 shows a common process flow diagram of an ethylene oxide

production plant. Beginning with ethylene and oxygen flow, which includes methane as

a dilutent, the mixture enters into the reactor. The reactor described was a shell and tube,

recycled plug flow, kerosene cooled reactor. In the reactor, the oxygen and ethylene

react to form ethylene oxide, carbon dioxide, and water. The hot effluent from the

reactor passes through a heat exchanger and is cooled down. Small amounts of methane

are added to the effluent stream at this time to dilute the mixture. The methane is mostly
inert, but some does react during the ethylene oxide production process which reduces the

inert’s concentration. The next unit operation is a water absorber. This removes the

ethylene oxide and water to be used in the production of ethylene glycol, or sold as

ethylene oxide at this point. The recycle stream is then purged of the inerts and scrubbed

to remove the carbon dioxide. Then, the recycle stream reenters the ethylene and oxygen

feed streams [4].

Diagram 1: Flow Diagram of Ethylene Oxide Production [4]

After the ethylene oxide is produced, an aqueous solution of ethylene oxide is fed

into a plug flow reactor. The reactor hydrolyses the ethylene oxide solution and forms

ethylene glycol. Common byproducts of this reaction are di-, tri-, and tetraethylene

glycols. After the solution exits the reactor, the water is removed by evaporation from

the mixture to form a recycle stream. The concentrated solution of mono, di-, tri-, and

tetraethylene glycol is fed into a stripper to remove any light impurities and the remaining
water after evaporation. The dehydrated solution is then fed into a series of distillation

columns to separate the various glycols [5].

Modeling a process such as the production of ethylene oxide and ethylene glycol

can save time and money for the companies beginning the process of construction of a

new industrial plant. Even though the process flow chart appears relatively simple, there

are many variables that need to be modeled and controlled to achieve the optimal

production of ethylene glycol. Feed streams of all of the reactors will have to be modeled

to determine optimal flow rates and ratios of reactants. One example is the ratio of

reactants, ethylene and oxygen, in the feed stream to the ethylene oxide reactor in

Diagram 1. Taking the initial step shown above in the flow chart, the ethylene, oxygen

and methane ratios can have dramatic influences on the final product. Also, one has to

weigh the product result versus the safety concerns of having potentially explosive

materials on site. Another variable is the reactor temperatures. Fuel needed to run

furnaces and reactors costs are major expenses of many industrial businesses. Modeling

a process will give a potential company an idea of how big to make the reactors, how

much fuel is required per day, and the number of operators needed to run the plant safely.

Modeling this process to maximize profit and minimize cost is the goal of all successful

businesses.

After the plant is built, modeling of the process can determine if improvements to

the production schedule can be made. Demand for ethylene glycol in the US in 1997 was

6.3 billion pounds rising to 6.5 billion pounds in 1998. Projected use in 2002 is 7.5

billion pounds. Between 1982-1997, the highest price that ethylene glycol sold for was

66 cents per pound. Current prices are approximately 20-21 cents per pound [6]. With
billions of pounds of ethylene glycol produced in the United States every year, every

penny saved by decreasing fuel costs and increasing yield means millions of dollars of

profit for the industry.

As stated earlier, one important control aspect and modeling concern of safely

working in the production of ethylene oxide, ethylene glycol and their by-products is to

know the individual properties of the components. Reactors in this process run at

approximately 250oC. This operating temperature is well above the flash point of

ethylene oxide and all of the glycols. Above the flash point temperature, vapor-air

mixtures are explosive within flammable limits as noted below. The LEL is the lower

explosive limit and the UEL is the upper explosive limit. Although, there is a slight to

moderate risk at room temperature if these chemicals are exposed to an open flame,

containers may explode if they are involved in a fire. Safely modeling a procedure can

decrease potentially hazardous conditions before production begins.

Boiling Point Flash Point Autoignition Temperature LEL UEL

Ethylene Glycol [7] 197.6 111 398 3.2 15.3
Diethylene Glycol [8] 245 117 229 1.7 10.6
Triethylene Glycol [9] 278 177 371 0.9 9.2
Tetraethylene Glycol [10] 276 141 *NA *NA *NA
Ethylene Oxide [11] 10.6 -18 429 3 100
*NA = not available
Table 1: Properties of Selected Products and Reactants

Up until this point, ethylene glycol produced from ethylene oxide is the only

production process discussed. Some alternate methods of producing ethylene glycol have

been conceived. According to one source, ethane can be reacted with water using a

hypothetical reaction to make ethyl alcohol [12]. By continuing this procedure one step

further, we can obtain ethylene glycol.

Ethylene glycol can also be readily prepared in the laboratory by refluxing ethylene

dichloride with a dilute solution of sodium carbonate [13].

One final interesting discover is that in the literature searches on the production of

ethylene glycol, most companies used a plug flow reactor to react the ethylene and

oxygen to form ethylene oxide, and the ethylene oxide and water to form ethylene glycol.

However, groups are modeling and experimenting on trying a different approach to the

production of ethylene glycol. They describe a reactive distillation column as a viable

alternative to plug flow reactors. “Reactive distillation is an innovating process which

realizes both distillation and chemical reaction into a solely unit. The underlying

motivation relies on the fact that industrial RDC [Reactive Distillation Columns]

processes may be operated at unstable operating conditions, which often corresponds to

optimal process performance. [14]” “Reactive distillation is potentially an attractive

process alternative to conventional liquid-phase chemical reaction processing for systems

that exhibit one or more of the characteristics: equilibrium limited chemical reactions,

exothermic reactions, poor raw materials usage due to selectivity losses, or excessive

flowsheet complexity. Commercial reactive distillation processes such as the Nylon 6,6

process, methyl acetate process and MTBE process have demonstrated reductions in

capital investment and/or energy consumption. [15]”

After analyzing ethylene glycol, looking at how ethylene glycol is produced, why

it should be modeled, and what are some proposed alternatives to common production

techniques, one should realize that this clear, colorless, and odorless substance is very

interesting in the eyes of researchers. Many possibilities lie in process alternatives, and

projected consumer usage makes it a profitable investment for industry. Modeling and
control systems are valuable tools and are used every day to maximize profit and

minimize costs for industry.

 References

[1] http://www.atsdr.cdc.gov/tfacts96.html

[2] http://www.chemexpo.com/news/profile981102.cfm

[3] http://www.us.thermal.alfalaval.com/process/ethylene_glycol.html

[4] http://www.owlnet.rice.edu/~ceng403/ethox97.html

[5] http://www.us.thermal.alfalaval.com/process/ethylene_glycol.html

[6] http://www.chemexpo.com/news/profile981102.cfm

[7] http://www.jtbaker.com/msds/e5125.htm

[8] http://msds.pdc.cornell.edu/msds/siri/msds/h/q131/q398.html

[9] http://www.mathesongas.com/msds/TriethyleneGlycol.htm

[10] http://www.mathesongas.com/msds/TetraethyleneGlycolDimethylEther.htm

[11] http://msds.pdc.cornell.edu/msds/siri/msds/h/q195/q116.html

[12] http://wey238ab.ch.iup.edu/olccii/student/eg.html

[13] http://www.ucc.ie/ucc/depts/chem/dolchem/html/comp/glycol.html

[14] Monroy-Loperena, Rosenda, et al. A robust PI control configuration for a high-

purity ethylene glycol reactive distillation column. Chemical Engineering Science
55 (2000) 4925-4937.

[15] Chen, Fengrong, et al. Simulation of kinetic effects in reactive distillation.

Computers and Chemical Engineering 24 (2000) 2457-2472.